There have been used a sound insulating material and a sound absorbing material for insulating undesired sounds and noises. As a sound absorbing material, unwoven cloth or a molded foam body which is soft and well absorbs sounds is generally used. The sound absorbing body as described above has no rigidity, and the sound insulating capability can not generally be expected.
For instance, Japanese Patent Laid-Open Publication No. 2000-206976 discloses the technology to improve the sound-absorbing capability and absorb sounds in a desired frequency range by providing through holes or semi-through holes each having a desired shape and size with a given pitch space in a foam sheet with both closed-cell air bubbles and open-cell ones existing therein. This patent describes, however, that through holes and semi-through holes are equally effective in absorbing sounds, and it can not be expected that this patent provides the sound insulating capability and rigidity.
On the other hand, as a sound insulating material, a material having high density and high rigidity and hardly vibrated by sonic waves is generally used. However, the sound insulating material as described above insulates sounds by reflecting sonic waves, so that the sound insulating material does not absorb sounds and generally the sound absorbing capability can not be expected.
With the sound absorbing material and sound insulating material based on the conventional technology, it is not possible to secure both the sound absorbing capability and sound insulating capability, and in order to secure both the sound absorbing capability and sound insulating capability simultaneously, complicated processes such as laminating a sound absorbing material and a sound insulating material together, so that the production process disadvantageously becomes complicated.
If it is tried to secure both the sound absorbing capability and sound insulating capability at the same time by laminating a sound absorbing material and a sound insulating material together, any of product characteristics such as the heat resistance, rigidity, light weight, and shape may be sacrificed. In addition, materials constituting the sound absorbing material and sound insulating material to be laminated together must be optimized, so that selection of the materials is also disadvantageously complicated.
For instance, Japanese Patent Laid-Open Publication No. Hei 8-244150 discloses a sound absorbing member formed by laminating a molded body prepared by heating and expanding a yarn-manufactured fiber-reinforced sheet (such as a KP sheet) and then compressing together with an ordinary resin molded body with an air layer therebetween. However, the holes provided in the papermaking method fiber-reinforced sheet completely penetrate through the sheet, so that the sound absorbing effect of the sheet itself is low and the sheet is very heavy.
Further, it is required to selectively absorb only sounds in a specified frequency range in some applications. For instance, sounds generated by an automobile engine include unpleasant ones and pleasant ones. Even if it is tried to absorb engine sounds with the conventional type of sound absorbing body, the conventional type of sound absorbing body can not selectively absorb sounds in a specific frequency range. Namely, there occurs the problem that not only unpleasant sounds but also pleasant sounds, e.g. an engine sound with a specific frequency, are absorbed.
For instance, Japanese Patent Laid-Open Publication No. 2000-52371 discloses providing semi-through holes on one surface of an injection-molded body having a skin layer on a surface thereof and a spring back structure therein, but does not suggest nor disclose that the desired sound absorbing capability can be achieved by setting a pitch space with a specific value.
An object of the present invention is to provide a sound absorbing body and a sound absorbing structural body capable of ensuring both the sound absorbing capability and sound insulating capability by integral molding without laminating a plurality of materials together and also selectively absorbing only unpleasant sounds, and a method of manufacturing the bodies.